The detonation of a radiological dispersal device (a dirty bomb), the deliberate damaging of a radioisotope production facility or indeed many other terrorist scenarios could result in multiple fragments of radioactive material being strewn over an area in relatively close proximity to one another, requiring rapid remediation. The presence of multiple radioactive sources in an area poses a problem in the isolation and identification of the radioactive material by first responders. First responders have difficulty localizing radiation sources in a multi-source environment due to the isotropic nature of most conventional radiation detection equipment, thus prolonging the time spent in potentially high dose-rate areas resulting in an increased dose received by the first responders. Current radiation detection equipment is isotropic in response, thus providing little directional information to the user. Even in the case of a single isolated radioactive source, the isotropic detector response will only give an indication of source location by examining dose rate trends—i.e. by moving physically closer to it, which extends time and thus dose to users. A detector capable of indicating direction coupled with dose-rate and spectroscopic information is not generally available, but is needed for improved nuclear radioisotope search missions with handheld instrumentation.
In close proximity to the source, the use of pancake probes to detect the associated beta emissions (very close to the ground or source), the use of collimated detectors, and the use of electronic methods are current solutions available to address this issue, however all have associated short-comings. At a distance, methods such as coded aperture imaging and Compton imaging have shown the ability to directionally identify a radioactive source's location in real-time.
The current solutions mentioned above have serious limitations. The use of pancake probes to detect the associated beta emissions (very close to the ground or source) will result in even more gamma dose to the responders; the use of collimated detectors will demand an unacceptable mass increase—i.e. the detector becomes to heavy to be considered portable; and the use of electronic methods, such as modified composite scintillator assembly systems (such as a phosphor sandwich), are not suitable at higher energies owing to particle range considerations. Monte Carlo simulations to investigate the feasibility of using a phosphor sandwich design clearly demonstrated their poor directional response and poor spectroscopic identification ability. Further to this, methods such as coded aperture imaging and Compton imaging require directional detection to be performed at a significant distance from the source, thus requiring the user to approach using conventional methods in order to remediate the scene. These systems are also typically large and bulky, currently transportable but not man-portable.
U.S. Pat. No. 5,665,970 by Stanley Kronenberg et al describes one type of directional radiation detector and imager where a pancake Geiger-Mueller counter is surrounded or sandwiched between two materials having different atomic numbers such as a thin layer of lead on one side and a layer of Lucite™ on an opposite side. The direction of a radiation source can then be calculated by rotating the detector but this takes time which increases the dose rate received by a person operating the detector.